Detector and dosimeter for neutrons and other radiation

ABSTRACT

A radiation detector and dosimeter is based on the fact that a sufficiently finely-dispersed liquid suspended in a host liquid of high viscosity or gel is stable at temperatures above its normal boiling point for long periods of time provided it is protected from contact with walls, or other types of initiators which can cause volatilization or vaporization of the droplets. Radiation, and particularly neutron radiation of sufficient energy and intensity on coming in contact with such droplets can trigger volatilization. The volume of vapor evolved can then serve as a measure of radiation intensity and dosage.

The Government has rights in this invention pursuant to Grant No. ENG75-02847 awarded by the National Science Foundation.

This is a division of application Ser. No. 820,059, filed July 28, 1977now U.S. Pat. No. 4,143,274.

BACKGROUND OF THE INVENTION

The need for radiation dosimeters, especially of the type to be carriedabout by individuals, is well recognized. The most popular of thedevices used at the present time is the photographic film badge in whichthe amount of radiation is determined by the number of tracks.

The track damage dosimeter is a device in which a very small track isleft in the material (e.g. mica, polycarbonate) when hit by radiation.To make the track large enough to measure, an etching solution isapplied, after which the number of tracks can be counted. The device,therefore, is indirect reading. It is not sensitive to gammas; it issensitive to neutrons. To make this device work, fissionable radiationfoils must be used that is, the wearer is exposed to radiation by thedevice which is designed to measure radiation. The exposure in one ofthese devices, despite a 2 mm lead shield, was 50 millirad per hour inthe mica under the badge. Such devices are indirect reading; also, theycannot readily be adjusted with respect to the minimum energy, i.e.threshold energy, to which they will respond.

Also available are devices which employ electrometers which aresensitive to gammas and/or both neutrons and gammas. These devices aredirect reading, but must be charged before use. They are fairlysensitive to vibration. (The electrometer may lose its charge therebygiving a false radiation reading). Devices of this type can be purchasedfor between $50.00 and $150.00 and thereafter need only be chargedon-site so that there are no recurrent costs.

The thermoluminescent detector, known as TLD, is a solid state device inwhich the active element, after exposure to nuclear radiation, willluminesce on being heated. The integrated light flux produced is ameasure of the radiation of a specific type to which the TLD has beenexposed. By selection of appropriate materials, the TLD can be made todistinguish between different types of radiation. However, just as isthe case with film badges, the TLD cannot be read directly, althoughon-site devices are available for reading the TLD.

The principle of radiation-induced nucleation of superheated liquids hasbeen used in the bubble chamber; such a device has a relatively largevolume of liquid under pressure. At the appropriate time the pressure isquickly dropped, thereby placing the liquid in the superheated state,radiation being incident on the sample. Bubbles form along the path ofradiation under properly controlled conditions. A serious disadvantageis that the bubble chamber can be kept at the low pressure only for verybrief periods of time before re-applying higher pressure, because bubblenucleation begins where it is not desired very quickly (on containersurfaces and in the bulk).

Another device depending on superheated liquids is the ultrasonic bubblechamber. Like the regular bubble chamber, this device is used fortracking radiation. However, here the pressure is rapidly cycledacoustically e.g. an acoustic standing wave can produce regions wherethe pressure oscillates locally. In these regions a vapor bubble mayform as radiation passes if the pressure has dropped sufficiently belowthe ambient pressure.

Skripov* has studied droplets of liquids which rise in a heated hostliquid. At a certain point the droplets become superheated. The higherthey rise, the more they become superheated. Eventually the dropletswill vaporize at the "limit-of-superheat" for the given pressure. But ifexposed to gammas, they will vaporize before they reach their"limit-of-superheat" temperature. Skripov has stopped these droplets byintercepting them with a glass dish. As long as a little of host liquidremains between the glass and the stopped droplet, the droplet will notvaporize prematurely.

As is evident, Skripov's research tool is not a practical device; itemploys a single droplet or perhaps a small number, but not many; itdoes not integrate the effect over time because the vapor is notretained; the host material in many of Skripov's experiments is sulfuricacid--which is certainly an impractical liquid for personnel dosimetry.

Skripov and his colleagues also used capillary tubes filled withsensitive liquid material which could be superheated by dropping thepressure (same reference). These were exposed to radiation sources, andthe time before vaporization could be measured. The capillary tubedevice is far less stable than the droplet method, i.e., the lifetime ofa superheated liquid in a capillary tube is relatively short because ofthe large surface area in contact with glass (or quartz) and the largevolume of liquid used. Heterogeneous nucleation, i.e. nucleation causedby solid particles or contact with walls, will cause vaporization of theliquid, making capillary tubes an impractical device as a radiationdetector.

An acoustic field in a liquid produces an oscillating pressure field,thereby sensitizing (i.e. superheating) and desensitizing the liquid atthe acoustic frequency. Also the high acoustic fields can be generatedlocally, away from container surfaces, thereby sensitizing only arelatively small volume. The interaction of radiation with liquid isnoted by an acoustic cavitation event,--a vapor bubble (or bubbles)grows and collapses, making an audible snapping sound, which can berecorded. Unfortunately, heterogeneous cavitation can occur unless theliquid is very carefully filtered. Also, a cavitation event may provideheterogeneous nucleation sites, i.e. the liquid must be refilteredbefore achieving an acceptably clean liquid. However, it is not apractical device for monitoring because of the down time in betweencavitation events.

As is evident then, although a variety of devices based on severaldifferent principles have been used for personnel monitoring, none ofthese has proved completely satisfactory. What is needed is a small,inexpensive device which is reliable and which can be monitored asfrequently as desired without terminating the usefulness of the specificinstrument as is the case when the film of a film badge is developed.

SUMMARY OF THE INVENTION

A liquid with a normal boiling point below the temperature at which itis to be used is dispersed in a host, the host consisting of a viscousliquid which may be termed a grease, or a soft gel. Either the grease orthe soft gel must be sufficiently yielding so that on vaporization of adroplet the resultant vapor will occupy a volume appropriate to theambient temperature and pressure. A suitable grease is a solution of ahigh polymer in water or gelatin softened with glycerol.

The dispersion of the sensitive (superheated) liquid in the host mediummay be carried out by withdrawing a specimen from a pressurized chamberusing a fine, hollow needle attached to a so-called "no-blow" syringe,and transferring the sensitive liquid through a septum into a specimenof the host medium in a pressurized chamber. The needle is moved aboutas the sensitive liquid is fed into the host medium, thereby producingfine droplets, preferably of size between 1 μm to 1 mm, with the chosensize depending on the particular application. In general, it isdesirable to add a small quantity of a preservative such as sorbic acidwhen the host medium is gelatin or is any other material subject tobacterial decomposition.

The device depends for its usefulness on the fact that the number ofinteractions per second is independent of the size of the droplets butdepends instead on the total volume of the sensitive liquid in thedevice. However, it is necessary that the droplet size and the totalvolume of liquid dispersed in the host in the device be scaled to theneutron density or flux expected to be encountered so that statisticalreliability will be provided.

Accordingly, an object of the present invention is a device fordetermining neutron dosage.

Another object of the present invention is a device for determiningneutron flux and integrating same.

A further object of the present invention is a device for determiningpersonal neutron dosage over a selected period of time.

An important object of the present invention is a device for determiningneutron dosage which does not require further development or treatmentand which can be inspected periodically for determining integratedneutron dosage.

A significant object of the present invention is a method of determiningneutron dosage both personal and in a given area.

A valuable object of the present invention is a method of determiningneutron dosage by the use of superheated liquids in which individualdroplets of superheated liquid are dispersed in individual compartmentsin a dosimeter.

Yet another important object of the present invention is a neutrondosimeter which provides information as to the distribution of energiesin a neutron flux.

Still another object of the present invention is a radiation dosimeterfor determining radiation flux and integrating same.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and thecomposition possessing the features, properties and the relation ofconstituents, and the article which possesses the characteristics,properties and relation of elements, all as exemplified in the detaileddisclosure hereinafter set forth, and the scope of the invention will beindicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to thefollowing description taken in connection with the accompanyingdrawings, in which:

FIG. 1 shows schematically how droplets of superheated liquid aredispersed in a host medium;

FIG. 2 shows apparatus for preparing film of a composition including ahost medium with droplets of superheated liquid dispersed therein;

FIG. 3a shows a side view of a badge-type detector based on theformation of a bubble, the volume of which is shown by graduations onthe package;

FIG. 3a' is a front view of the detector of FIG. 3a;

FIG. 3b is a side view of another embodiment of the badge-type detectorof FIG. 3a;

FIG. 3b' is a front view of the embodiment of FIG. 3b;

FIG. 4a is a detector in cylindrical form prior to use, said detectorbeing based on the formation of a bubble, the size of which can be readon graduations;

FIG. 4b is the detector of FIG. 4a subsequent to use;

FIG. 5a is another embodiment of the invention prior to use, the neutrondosage being indicated by the extent to which a float rises in abuoyancy liquid;

FIG. 5b is the detector of FIG. 5a subsequent to use;

FIG. 6a is a further embodiment (prior to use) in which thedetermination of dosage is based on the volume expansion of a flexiblepacket;

FIG. 6b is the embodiment of FIG. 6a subsequent to use;

FIG. 7 is a dosimeter in which the dosage is indicated by a colorchange;

FIG. 8a is a front view of a dosimeter suitable for detection of neutronradiation over a substantial area;

FIG. 8b is a sectional view of the dosimeter of FIG. 8a;

FIG. 9 indicates how embodiments of the present invention may becombined to yield an energy spectrum for radiation;

FIG. 10 is a further embodiment of apparatus for determining an energyspectrum;

FIG. 11 is an embodiment which is sensitive both to neutron radiationand to charged particle radiation; and

FIG. 12 shows in section a sheet of host medium containing indentationseach of which holds droplets of superheated liquid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally speaking, the present invention is based on a superheatedliquid dispersed in a soft host medium. The superheated liquid generallyhas a boiling point at 760 mm of mercury between about -40° C. and +10°C. where the liquid is to be a component in a device used for personneldosimetry. In general, for any specific application, the boiling pointof the liquid must be lower than the use temperature at the ambientpressure, thus ensuring that the liquid is superheated, and therefore,"sensitive".

There is a limiting temperature to which any liquid can be superheated,this limiting temperature being known as the "homogeneous nucleationlimit". A liquid such as Freon 114 (C₂ Cl₂ F₄, made by Dupont) and alsomade by Union Carbide under the tradename Ucon 114, has a boiling pointof 3.8° C. but a homogeneous nucleation limit of about 110° C.Calculations show that for a volume of 0.005 ml of sensitive liquid,homogeneous nucleation will take place in about 17 minutes if thetemperature of the liquid is 1° C. below the homogeneous nucleationlimit. This theoretical waiting time rises to one-half year for 2° C.below this limit and one million years for 3° C. below this limit.Consequently, to avoid homogeneous nucleation, the compositions ofsubject invention should be used at temperatures at least 2° C. andpreferably 3° C. below the corresponding homogeneous nucleation limitingtemperatures.

Nucleation can also occur heterogeneously, i.e., as the result ofpre-existing bubbles, solid impurities and contacts with the wall of acontainer.

Other suitable liquids are butane, 1-butene and cis-2-butene.

Where the compositions of subject invention are to be used for thedetection of incident radiation, the energy of the incident radiationmust be above a specific threshold or minimum level, at which pointthere is a measurable rate of interaction. For neutron radiation, theprobability of interaction of a single neutron with a nucleus of an atomis extremely small, but with a sufficiently large number of nuclei, asdetermined by the volume of sensitive material, the probability ofinteraction can be brought to a useful range. The number of interactionsper second φ is given by the formula

φ=cψVσ

where V is the total volume of the sensitive liquid and σ and c are thecross-section for interaction and a constant respectively, for the givenliquid. The cross-section for interaction is determined by directmeasurement. Typical cross-sections are of the order of 10⁻²⁴ cm² (1barn) for neutrons in the energy range of 1-10 MeV. ψ is the flux ofneutrons per square centimeter per second.

It should be noted that the number of interactions per second dependsupon the total volume of the sensitive liquid and is independent of thedegree to which the volume is divided into drops. Thus, if the volume isdivided into n drops, the number of interactions for each drop persecond is φ/n. If a dosimeter is designed so that only a small fraction,f, of droplets will vaporize in the use time of the dosimeter, then theinteraction rate will remain very close to φ. Consequently, for a givenliquid and for a given incident flux of neutrons, the interaction ratewill depend only on the total volume of sensitive liquid. Assuming thereare enough interactions for statistics to be valid, then the totalvolume of vapor generated by interactions will be proportional to theincident flux. Moreover, the sensitive droplets need not be uniform insize, even if the total number of interactions is small compared to thetotal number of droplets. As a typical example, for Freon 114 at roomtemperature, the interaction rate is about 3.5×10⁻³ Vψ where V is in cm³and ψ is the number of neutrons per square centimeter per second. For aflux of 10 neutrons/cm² /s, which is about the maximum allowable ratefor radiation workers and for the case where V=1 cm³, the interactionrate is about 0.035/s or about two per minute. The actual volume ofsensitive material required for a practical dosimeter depends on thesensitive liquid used, the desired interaction rate, the desired timeduring which the dosimeter is to be used and the expected incident flux.

Still considering Freon 114 at room temperature, the interaction rate ψfor 1-10 MeV neutron-induced vaporization is equal to 3.5×10⁻³ ψV. Tocalculate once more the volume of composition necessary for a fastneutron flux of 10 neutrons/cm² /s, it may be assumed that the rate ofone interaction per 10 minutes (600 seconds) would be an acceptable rate(about 50 interactions per 8 hour day). Then, using the above formula,it is found that V=0.05 cm³.

In order to maintain the same interaction rate over the duration T ofthe use of the dosimeter, the number of droplets should be largecompared to the total number of interactions (φ×T), so that thesensitive liquid volume remaining is close to the original volume. Then,continuing the above example, for a dosimeter to be effective for oneweek (40 hours), the number of droplets should be greater than about5×φ×T=1200 droplets.

The average volume of a drop is equal to the total sensitive volumedivided by the total number of drops. For the present example this worksout to 4.17×10⁻⁵ cm³. The diameter of such a drop is 0.43 mm. Ingeneral, the droplet size does not influence the interaction rate, therate depending on the total volume of sensitive liquid in the device.Also, the smaller the diameter of the drop, the smaller the resultingvapor volume. When the drop is vaporized, droplet sizes can range fromabout 1 μm to 1 mm, with the chosen size depending on the particularapplication.

To increase the statistical reliability of results, it is desirable thatdroplet size be relatively uniform. Where droplets are dispersed bysyringe, droplets of uniform size can be produced by moving the tip ofthe syringe after a selected volume of liquid has been extruded fromsaid tip. As a further means of increasing the statistical reliability,the rate of triggering by a unit volume of sensitive liquid can beincreased. Specifically Freon 12 is far more sensitive than Freon 114.Also, a greater volume of sensitive liquid will produce more vapor in aselected time interval. Thus, 2 ml of Freon 12 exposed to a flux of 10neutrons/cm² /sec. would give an interaction every 2 seconds as comparedwith Freon 114 which would produce interactions at a rate of one every600 seconds.

To prepare a composition in accordance with the present invention, asensitive liquid is selected for dispersion in an appropriate hostmedium. The host medium should be a material of sufficiently highviscosity so that during the dispersion process the droplets formed willbe encapsulated by the host medium and will remain encapsulated whetherthe medium retains its initial viscosity or the viscosity increases asis the case with solutions of gelatin. A small quantity, i.e., fromabout 0.05% to about 0.3%, of a dispersing agent, or emulsifying agentmay be used to facilitate dispersion of the sensitive liquid in the hostmedium. The presence of such a dispersing agent makes it possible toprepare satisfactory compositions with hosts of relatively lowviscosity. Polymers which coat the droplets also make it possible toprepare low viscosity compositions. The process of manufacture isillustrated by the following examples.

EXAMPLE I Gelatin Gels

To 100 parts of distilled water, 4% by weight of Knox brand gelatin wasadded. The mixture was heated until the gelatin dissolved. To theresultant quantity of liquid gelatin an equal weight of glycerol wasadded. Ratios of glycerol to gelatin solution ranging from 0.5 to 2 havebeen found useful. Also, gelatin solutions ranging in concentration from3% to 5% have been found useful. The gelatin solution should containfrom about 0.1% to 0.5% of sorbic acid or other suitable preservative.

The gelatin-glycerol solution, henceforth termed "composition" andindicated by the reference numeral 11 in FIG. 1 was placed in a vial 12in a pressure chamber 13 sealed with a cover 14 and pressurized.

A sensitive liquid, in this case Freon 114 and indicated by thereference numeral 16 was stored in a pressurized container 17 alsohaving a septum 18 in a cover 19. The pressure in the container wasmaintained at a level sufficient to prevent the liquid from boiling.

A syringe of the no-blow type was inserted into the reservoir and avolume of liquid was withdrawn therefrom, the syringe being constructedwith a valve thereon to maintain the liquid under pressure duringstorage in the syringe and transfer. The needle 21 of the syringe wasinserted through the septum 22 in pressure chamber 13 and into hostmedium 11 in vial 12. The syringe valve 23 was opened and droplets ofthe sensitive liquid were injected into host medium 11. During theinjection, the needle was manipulated so that droplets would be formedat reasonable intervals, it being desired to avoid placing the dropletsclose together since triggering of one drop by volatilization of anothermay occur when the spacing between the droplets is small.

The syringe needle was withdrawn after injecting about 50 drops of0.5-1.0 mm diameter into the host medium. The pressure in the chamberwas then dropped to atmospheric and the vial was removed. The dropletsin the host medium at this point became superheated since the sensitiveliquid was at a temperature above its normal boiling point, that is, theboiling point at atmospheric pressure.

Generally, the gelatin sets prior to dispersing the droplets therein,but the gel is sufficiently soft as the result of the addition of theglycerol so that the needle can be moved about therein and droplets canbe formed in the gel without causing permanent disruption of the gel.

Most important, the gel is sufficiently soft so that on vaporization ofa droplet by interaction with a neutron, the vapor formed occupies avolume corresponding to the ambient temperature and pressure.Furthermore, if the droplet is close to the surface of the gelatin, thebubble formed will break the surface of the gelatin and emerge into thespace above the gel.

EXAMPLE II Aquasonic 100 Gel Preparations (Parker Laboratories,Irvington, N.J.

Aquasonic 100 is a water-soluble gel containing a vinyl polymerdissolved in water. To the gel were added varying amounts of waterand/or glycerol. A suitable host medium consists of equal quantities (byweight) of glycerol and gel. Another suitable host medium consists of 1part gel, 0.25 parts water and 0.25 parts glycerol, all parts being byweight. Yet another suitable host medium consists of 1 part Aquasonic100, 2 parts of glycerol and 1/2 part of water, all parts being byweight. The ratio of glycerol to Aquasonic 100 can be between about 0:1and 2:1.

The Aquasonic 100 compositions were produced by vigorous mixing of theliquids until a uniform composition was formed. During the process somebubbles were produced. These were eliminated by outgassing under vacuum.They can also be eliminated by subjecting the compositions to hydraulicpressurization to 5,000-10,000 psi for about 20 minutes.

Aquasonic 100 solutions with added glycerol and with or without addedwater were placed in the pressure chamber of FIG. 1 and droplets ofFreon 114 were dispersed therein in the same manner as described withrespect to the gelatin gels. The resultant compositions proved to besatisfactory for detection of neutron radiation.

EXAMPLE III Miscellaneous

A hair-setting gel sold under the name of "Dippity Doo" (Gillette Co.)was placed in the pressurizable chamber and Freon 114 was dispersedtherein. The product proved to be satisfactory for detection of neutronradiation.

Similar results can be obtained using a variety of greases such as thosebased on the polyethylene oxides. It is only necessary that the dropletbe encapsulated in a microscopically smooth material which will notinitiate vaporization, and which will give way when radiation hasinitiated vaporization.

The stability of the product can be greatly enhanced by placing thecomposition in a liquid-filled plastic bag and hydraulicallypressurizing at a sufficiently high pressure for a sufficient period oftime to eliminate all minute gas bubbles. A suitable pressure is 10,000psi and a suitable time is 20 minutes at this pressure.

EXAMPLE IV

Freon 114 is denser than water, and when injected into gelatin which hasnot as yet set the sensitive material will be encapsulated in thegelatin and will fall to the bottom of the vial and aggregate there. Thedroplets do not coalesce because they are coated by the gel. Afterinjecting the droplets, the chamber pressure is reduced to atmosphericpressure and the vial removed. The droplets do not vaporize and remainstable. The contents of the vial, including the droplets, can then bepoured into a dish from which the individual droplets can be pipettedinto any desired substrate. Thus, considering a substrate which is asheet having a series of indentations, a single droplet can be pipettedinto each indentation. To avoid premature droplet vaporization, thesubstrate is kept at a temperature below the boiling point of thesensitive liquid. Once the substrate is prepared, the auxiliaryprocedure of hydraulic pressurization will assure long term stability ofthe drops.

EXAMPLE V Low Temperature Dispersal of Drops in Host Material

In order for this procedure to be effective, it must be possible to coolthe gel below the normal boiling point of the sensitive liquid withoutfreezing the gel.

The sensitive liquid and the gel are brought to a temperature below thenormal boiling point of the sensitive liquid. The gel is placed in asuitable holder or on a suitable substrate. A syringe is filled with thesensitive liquid and droplets of the sensitive liquid are injected ontothe substrate at desired positions. Once the droplets are covered withgel, the resultant dispersion can be allowed to warm up to the desiredoperating temperature and fabricated into a suitable package.

EXAMPLE VI

A suitable composition can be prepared by the use of emulsificationequipment. Thus, as shown in FIG. 2, tank 26 contains a host medium andtank 27 contains the sensitive liquid to be dispersed. Both tanks arepressurized and the materials are fed by the use of a pump 28 or bygravity and pressure drop to chamber 29 containing an emulsificationrotor 31 driven by motor 32 through shaft 33. The dispersion emergesthrough slot 34 in the form of a film 36 on metal sheet conveyor belt37. The sheet is chilled to cause rapid setting of the film.

Sensitive dispersions made in accordance with the methods described weretested against a plutonium-beryllium source emitting about 1.85×10⁶neutrons/second with energies below 10 MeV. Tests were also made withgamma sources including a cobalt 60 source (having energies of 1.33 and1.77 MeV) and accelerator sources (energies at 6 MeV and at 25 MeV). Anelectron source from an accelerator had energies at 7, 10, 13, 16, 19,22, 25 and 32 MeV.

Freon 12 (CCl₂ F₂), also known as Ucon 12, is superheated by 50° C. atroom temperature. It showed great sensitivity to the neutron source, butno response to the strong cobalt 60 source, and extremely smallinteraction rates for 6 MeV gamma rays and a high interaction rate for25 MeV gamma rays. This liquid showed only a low interaction rate forelectrons, even at high energies.

Freon 114 also known as Ucon 114, (C₂ Cl₂ F₄) is superheated by 19° C.at room temperature. It is sensitive to neutrons but at a lowerinteraction rate than Freon 12, has no sensitivity to the cobalt 60 orthe 6 MeV gamma source, and only a low interaction rate for the 25 MeVgamma source. It has a very low interaction rate for electrons above 13MeV.

Butane is superheated by about 23° C. at room temperature. It was testedonly against 6 MeV gamma rays, but gave virtually no response.

Following are a number of compounds suitable for use at normal ambienttemperatures, the differences in boiling point providing differences inthe degree of superheat and thus a range of sensitivities:

    ______________________________________                                        Compound        Boiling point, °C. 760 mm Hg                           ______________________________________                                        C.sub.2 Cl.sub.2 F.sub.4 sold as Ucon 114                                     by Union Carbide and Freon                                                    114 by Du Pont  3.8                                                           CCl.sub.2 F.sub.2 sold as Ucon 12                                             or Freon 12     -29.8                                                         n-butane, C.sub.4 H.sub.10                                                                    0.5                                                           1-butene, C.sub.4 H.sub.8                                                                     -6.3                                                          cis-2-butene, C.sub.4 H.sub.8                                                                 3.7                                                           2,2-dimethyl propane, C.sub.5 H.sub.12                                                        9.5                                                           ______________________________________                                    

Solutions of a sensitive compound in another sensitive compound are alsouseful, such solutions providing control over the degree of superheat.

As is evident, these results indicate that superheated liquids arepreferentially sensitive to neutrons and insensitive to gamma rays atenergies below 6 MeV, a very desirable feature in a neutron detectordevice. However, where radiation of other types to which the superheatedliquid is sensitive is present and it is desired to distinguish betweenneutron radiation and such other types, shielding may be incorporated ina device within the scope of the present invention. Thus, chargedparticles are effectively blocked by a layer of glass and a layer oflead sheet will prevent triggering by gamma rays.

A wide variety of dosimeters for various applications can be prepared.FIGS. 3a and 3a' show a badge-type detector, indicated generally by thereference numeral 41, which contains a sheet 42 of a compositionconsisting of a sensitive liquid dispersed in a host medium which is agel. Desirably, the gel has therein a preservative such as sorbic acidin small quantity when the gelling agent is subject to bacterial attack.Container 45 is preferably of glass, the wall of the container servingas a shield against charged particles.

When a neutron interacts with a droplet and initiates vaporization, thevapor escapes into vapor space 43 above inert liquid 44. In therectangular configuration of FIG. 3a the volume of the separated vaporbubble 43 is measured by means of graduations 46 on the exterior of thepackage.

A somewhat similar structure is shown in FIGS. 3b and 3b' except thatthe package is circular in format and must be placed on a horizontalsurface for reading. The graduation marks are concentric circles, andmay be such that they relate bubble volume to radiation dose.

Both of these embodiments depend upon the fact that when the dropletvaporizes it vaporizes in a direction which is perpendicular to the hostsubstrate, passing into the immiscible liquid 44.

A cylindrical embodiment having a "pen" format is shown in FIGS. 4a and4b wherein the composition is in the form of a sheet 48 rolled up and isa cylinder as shown. The sheet is immersed in inert liquid 49 fillingflexible transparent plastic cylindrical bag 51 which is initiallycollapsed as shown in the left-hand portion of FIG. 4. The assembly iscontained in a transparent graduated shield 52, the shield being rigid.After exposure to radiation, the vapor formed by volatilization ofradiation-triggered droplets and the degree of expansion can be readagainst the graduation marks on the shield.

FIGS. 5a and 5b show an embodiment resembling a hydrometer. Thesensitive liquid dispersed in the host, indicated by the referencenumeral 54, is in a plastic bag which can expand. Affixed to the top ofeach bag is a vertical rod bearing graduation marks, and the entireassembly is immersed in an inert liquid 57. The density of the liquid isselected so that the plastic bag containing the sensitive compositionfloats with most or all of the rod portion thereof submerged prior toexposure to radiation. Preferably, the top of rod 56 is initially at thesame height as the surface 58 of inert liquid 57.

When the device is brought into a region of neutron irradiation,droplets of sensitive liquid are vaporized and the buoyancy of the bagincreases, causing the bag to rise so that the rod protrudes above thesurface of the inert liquid. From the distance which rod 56 protrudesabove the surface of the liquid, the volume of liquid vaporized can becalculated and from this volume the total quantity of neutron fluxintegrated over the radiation time becomes known.

The embodiment shown in FIGS. 6a and 6b is of the "thermometer" type. Acomposition 61 containing droplets of superheated liquid in a hostmedium is in a plastic expandable bag 62, bag 62 being anchored to thebottom of container 60 by cement 65. On exposure to radiation dropletsvaporize and the bag expands, displacing inert liquid 63 up into narrowportion 64 of container 66. Narrow portion 64 is graduated and from theposition of the surface 67 within the graduated portion, the volume ofvapor and the total quantity of radiation exposure can be calculated.

FIG. 7 shows an embodiment in which the droplets of sensitive liquid inthe film 69 of sensitive composition contain a source of color such as adye or an acid or a base. Above the film of sensitive composition is alayer of inert liquid 71 which may contain an acid-base indicator.Alternatively, liquid 71 may be a solvent for the dye, showing itscolor. The color can be observed through transparent shield 72. Affixedto the back of the assembly may be a color chart 70 which relates theradiation dose to color.

FIGS. 8a and 8b show a radiation dosimeter applicable to measuringradiation dose over a relatively large area. A compartmentalizedcontainer 76 shows radiation dosage at each compartment 77, using atechnique such as that shown in FIGS. 3a and 3b where the separation ofa bubble is a measure of radiation dosage at each compartment.Partitions 78 between compartments are formed by standard sealingtechniques.

It has been found that the minimum energy necessary to triggervaporization of a droplet of a superheated liquid depends upon thenature of the liquid. Consequently, by utilizing a plurality of packetsof superheated liquid, each packet containing a different superheatedliquid, an energy spectrum can be obtained. The same effect can beachieved by having several dosimeters of the same sensitive liquid eachat a different pressure. The highest pressure must be less than thevapor pressure of the liquid at the temperature of use of the dosimeter,since otherwise, the liquid will not be superheated, and hence will beinsensitive to radiation. Within this limit, the higher the pressure,the greater the energy threshold. FIG. 9 shows such an assembly usingthe hydrometer type of instrument. All of the containers are connectedto a single high pressure source 81 through individual pressureregulators 82, each container being also fitted with a pressure gauge83. Each regulator is set at a different value, thereby settting theminimum energy which a neutron must have in order to interact with theparticular sensitive liquid selected.

FIG. 10 shows an energy spectrometer based on temperature difference.Each of the buoyancy-type devices is maintained at a differenttemperature so that the degree of superheat is different for eachdevice. In the spectrometer shown in FIG. 10, device 91 is at thehighest temperature and devices 92, 93 and 94 are at successively lowertemperatures. Since the degree of superheat is greatest for device 91which is at the highest temperature, the frequency of interaction willbe greater and the expansion of bag 96 will exceed those of bags 97, 98and 99.

As is obvious, other types of detectors than the buoyancy-type wouldalso be effective for the purpose indicated. Further, as aforenoted, anenergy spectrometer can be used on the basis of the use of differentsuperheated liquids. Particularly advantageous is the use of twomiscible liquids mixed in varying proportions where the two liquids havewidely different threshold energies for triggering.

FIG. 11 shows an embodiment of the present invention which is sensitiveboth to neutrons and to charged particles. A layer 101 of disperseddroplets in a host has a flexible membrane 102 thereover and a thinplastic sheet 103 thereunder. The plastic enclosure is held in atransparent shield 104 of a material such as glass, said shield beingcontinuous on the top portion thereof and perforated on the lowerportion thereof, said lower portion being indicated by the referencenumeral 106. Neutrons can enter the sensitive composition 101 eitherfrom above or below and charged particles can enter through theperforations in the lower face of the device.

In the embodiment of FIG. 12, a substrate 107 has indentations 108therein in each of which there are a plurality of droplets 109 of asensitive liquid. The droplets are embedded in a host substance 111which is either a grease or a soft gel. Such a sheet is useful fordetermining area distribution of neutron flux. It may be shielded fromgamma rays, dust, etc., by any of the means made obvious in the otherembodiments disclosed herein.

It has been noted that triggering of a droplet is accompanied by anaudible sound. The sound can be amplified by appropriate apparatus tosignal entry of the wearer into an area of high neutron flux. Also, asis evident, any of the devices disclosed herein can readily be modifiedto sound an alarm when a specific volume of vapor has been generated.Such an apparatus is shown schematically in FIG. 11 in which sensor 100is connected to amplifier-speaker 110 to signal each volatilization of adroplet. In addition, sensor 100 can be connected to integrator 105 tosound an alarm by means of amplifier-speaker 110 when a specified dosagehas been received. Also, an alarm can be emitted when the volume ofvapor evolved reaches a selected value.

It should be noted that so far as detection of neutrons is concerned,neither the material of which the containers are constructed nor thenature of the host composition is significant. The reason is that forwater and for paraffin, for instance, which are among the most effectiveabsorbers of neutron radiation, there is negligible shielding effectwhere the thickness of the material under consideration is less thanabout 3 cm.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in the above composition of matter andin the article including said composition of matter, without departingfrom the spirit and scope of the invention, it is intended that allmatter contained in the above description shall be interpreted asillustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A composition including droplets of superheatedliquid encapsulated in a host immiscible therewith, said host being amember of the group consisting of viscous liquids, greases and softgels, said viscous liquid and grease being of high enough viscosity toprotect said droplets from initiation of vaporization by contact with awall, and said gel being soft enough to allow essentially the fullvolume change corresponding to the vaporization of any droplets at theambient temperature and pressure about said gel, said composition beinguseful in the detection of radiation.
 2. The composition as defined inclaim 1, wherein said gel consists essentially of gelatin, water andglycerol.
 3. The composition as defined in claim 1, wherein said gelconsists essentially of 100 parts of 3% to 5% gelatin in water, and 50to 200 parts of glycerol, all parts being by weight.
 4. The compositionas defined in claim 1, wherein said gel is a high polymer dissolved inwater in combination with glycerol as said softener.
 5. A composition asdefined in claim 1, wherein said superheated droplets are of a member ofthe group consisting of C₂ Cl₂ F₄, CCl₂ F₂, n-butane, 1-butene,cis-2-butene, 2-2 dimethyl propane, and miscible combinations thereof.6. The composition as defined in claim 1, wherein said droplets rangefrom about 1 μm to about 1 mm in size.
 7. The composition as defined inclaim 1, wherein said composition is in the form of a film sufficientlythin so that expansion of a droplet on vaporization will take placeprincipally forward and out of the surface of said film.
 8. Thecomposition as defined in claim 1, further comprising a coloring meansin said superheated liquid for producing a color in a second liquidcontiguous with said composition, said coloring means being a memberselected from the group consisting of dyes, acids and bases, said secondliquid being a solvent for said member when said member is a dye, andcontaining in solution therein an acid-base indicator when said memberis an acid or a base.
 9. A method of preparing a stable compositionincluding a superheated liquid, comprising the step of dispersing aliquid in a host selected from a group consisting of viscous liquid,greases and soft gels, said liquid being immiscible with the members ofsaid group, the boiling point of said liquid at the pressure at whichsaid composition is to be used being lower than the ambient temperatureat said pressure, said composition being useful in the detection ofradiation.
 10. The method as defined in claim 9, wherein said dispersalis carried out at approximately room temperature at superatmosphericpressure.
 11. The method as defined in claim 9, wherein said dispersalis effected at a temperature below the normal boiling point of saidliquid, said host being liquid at said temperature.
 12. The method asdefined in claim 9, wherein said step comprises the substeps ofwithdrawing a volume of said liquid from a pressurized reservoir,dispersing said volume of liquid in said host in a chamber at a pressuregreater than the vapor pressure of said liquid at the ambienttemperature of said host.
 13. The method as defined in claim 12, furthercomprising the substep of releasing the pressure over said host.
 14. Themethod as defined in claim 9, wherein said dispersal is effected by anemulsifier.
 15. The method as defined in claim 12, wherein saiddispersal is effected by a syringe needle inserted into said reservoirthrough a septum for withdrawing said volume and into said host througha septum in said chamber, said needle being moved about to form dropletsof liquid in said host as said liquid is dispersedly injected into saidhost.
 16. The method as defined in claim 9, wherein said liquid isinjected dropwise by syringe needle means into each of the compartmentsin a compartmented sheet holding host in said compartments.
 17. Themethod as defined in claim 9, wherein said dispersing of said liquid ina host is effected in a vessel having an exit slit therein and furthercomprising the step of feeding the resulting dispersion through saidslit in thin sheet form onto a chilled moving belt.
 18. The method asdefined in claim 9, further comprising the step of hydraulicallypressurizing said composition at a pressure and for a time long enoughto cause solution of any gas bubbles present, thereby minimizing thepossibility of triggering vaporization of said droplets by contact withgas bubbles.
 19. The composition as defined in claim 1, wherein saidhost consists essentially of a high polymer dissolved in water.
 20. Thecomposition as defined in claim 19, wherein said high polymer ispolyethylene oxide.
 21. The composition as defined in claim 1, whereinsaid dispersed, superheated liquid comprises at least two differentcompounds miscible with each other for controlling the degree ofsuperheat in use.